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Introductory Chemistry , 2 nd Edition Nivaldo Tro

Introductory Chemistry , 2 nd Edition Nivaldo Tro. Chapter 2 Measurement and Problem Solving Part 1: Measurements. What is a Measurement?. quantitative observation of a property comparison to an agreed upon standard every measurement has a number and a unit. Parts of a Measurement.

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Introductory Chemistry , 2 nd Edition Nivaldo Tro

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  1. Introductory Chemistry, 2nd EditionNivaldo Tro Chapter 2 Measurement and Problem Solving Part 1: Measurements

  2. What is a Measurement? • quantitative observation of a property • comparison to an agreed upon standard • every measurement has a number and a unit Tro's Introductory Chemistry, Chapter 2

  3. Parts of a Measurement • The unit tells you what property of and standard you are comparing your object to • The number tells you • what multiple of the standard the object measures • the uncertainty in the measurement • A number without a unit is meaningless because it doesn’t tell what property is being measured. Tro's Introductory Chemistry, Chapter 2

  4. Scientists measured the average global temperature rise over the past century to be 0.6°C • °C tells you that the temperature is being compared to the Celsius temperature scale • 0.6°C tells you that • the average temperature rise is 0.6 times the standard unit • the uncertainty in the measurement is such that we know the measurement is between 0.5 and 0.7°C Tro's Introductory Chemistry, Chapter 2

  5. an atom’s average diameter is 0.000 000 000 3 m Scientific Notation • A way of writing very large and very small numbers • Writing large numbers of zeros is confusing • not to mention the 8 digit limit of your calculator! • Very easy to drop or add zeros while writing The sun’s diameter is 1,392,000,000 m Tro's Introductory Chemistry, Chapter 2

  6. an atom’s average diameter is 3 x 10-10 m Scientific Notation • Each decimal place in our number system represents a different power of 10 • Scientific notation writes numbers so they are easily comparable by looking at powers of 10 • Has two parts: 1. coefficient = number with values from 1 to 10. 2. exponent = power of 10 the sun’s diameter is 1.392 x 109 m Tro's Introductory Chemistry, Chapter 2

  7. an atom’s average diameter is 3 x 10-10 m Exponents = Powers of 10 • When exponent on 10 is positive, it means the number is that many powers of 10 larger • sun’s diameter = 1.392 x 109 m = 1,392,000,000 m • when exponent on 10 is negative, it means the number is that many powers of 10 smaller • avg. atom’s diameter = 3 x 10-10 m = 0.0000000003 m the sun’s diameter is 1.392 x 109 m Tro's Introductory Chemistry, Chapter 2

  8. exponent 1.23 x 10-8 decimal part (coefficient) exponent part Scientific Notation • To compare numbers written in scientific notation • First compare exponents on 10 • If exponents equal, then compare decimal numbers (coefficient) 1.23 x 105 > 4.56 x 102 4.56 x 10-2 > 7.89 x 10-5 7.89 x 1010 > 1.23 x 1010 Tro's Introductory Chemistry, Chapter 2

  9. Writing Numbers in Scientific Notation • Locate the decimal point • Move the decimal point to the right of the first non-zero digit from the left • Multiply the new number by 10n where n is the number of places you moved the decimal point • if the number is ³ 1, n is +; if the number is < 1, n is - Tro's Introductory Chemistry, Chapter 2

  10. Writing a Number In Scientific Notation 12340 • Locate the Decimal Point 12340. • Move the decimal point to the right of the first non-zero digit from the left 1.234 • Multiply the new number by 10n • where n is the number of places you moved the decimal pt. 1.234 x 104 • If the number is ³ 1, n is+; if the number is < 1, n is - 1.234 x 104 Tro's Introductory Chemistry, Chapter 2

  11. Writing a Number In Scientific Notation 0.00012340 • Locate the Decimal Point 0.00012340 • Move the decimal point to the right of the first non-zero digit from the left 1.2340 • Multiply the new number by 10n • where n is the number of places you moved the decimal pt. 1.2340 x 104 • if the number is ³ 1, n is +; if the number is < 1,nis- 1.2340 x 10- 4 Tro's Introductory Chemistry, Chapter 2

  12. Writing a Number in Standard Form 1.234 x 10-6 • since exponent is -6, make the number smaller by moving the decimal point to the left 6 places • if you run out of digits, add zeros 000 001.234 0.000 001 234 Tro's Introductory Chemistry, Chapter 2

  13. Scientific Notation: Example 2.1 • The U.S. population in 2004 was estimated to be 293,168,000 people. Express this number in scientific notation. • 293,168,000 people = 2.93168 x 108 people Tro's Introductory Chemistry, Chapter 2

  14. 1.23 Enter 1.23 +/- -1.23 Press EXP -1.23 00 Press -1.23 03 Enter 3 +/- -1.23 -03 Press Entering Scientific Notation into a Calculator -1.23 x 10-3 • Enter decimal part of the number • if negative press +/- key • (–) on some • Press EXP • EE on some • Enter exponent on 10 • press +/- key to change exponent to negative Tro's Introductory Chemistry, Chapter 2

  15. use ( ) liberally!! type in decimal part of the number if negative, first press the (-) Enter exponent Enter exponent number if negative, first press the (-) Press 2nd, then EE -1.23E (-) Press -1.23E- Enter 3 -1.23E-3 Entering Scientific Notation into a TI-83 Calculator -1.23 x 10-3 (-) – Press Enter 1.23 –1.23 Tro's Introductory Chemistry, Chapter 2

  16. Exact Numbers vs. Measurements • Exact numbers: sometimes you can determine an exact value for a quality of an object • often by counting • pennies in a pile • sometimes by definition • 1 in (inch) is exactly = 2.54 cm • Measured numbers are inexact = obtained using a measuring tool, i.e. height, weight, length, temperature, volume, etc. Tro's Introductory Chemistry, Chapter 2

  17. Uncertainty in Measurement • Measurements are subject to error. • Errors reflected in number of significant figures reported. • Significant figures = all numbers measured precisely plus one estimated digit. • Errors also reflected in observation that two successive measurements of same quantity are different.

  18. Uncertainty in Measurement Precision and Accuracy • Accuracy = how close measurements are to “correct or true” value. • Precision= how close several measurements of same quantity are to each other.

  19. Precision and Accuracy Measurements can be a) accurate and precise b) precise but inaccurate c) neither accurate nor precise. a b c Prentice Hall

  20. 45.872 estimated certain Reporting Measurements • Measurements are written to indicate uncertainty in the measurement • The system of writing measurements we use is called significant figures • When writing measurements, all the digits written are known with certainty except the last one, which is an estimate 45.872 Tro's Introductory Chemistry, Chapter 2

  21. Estimating the Last Digit • For instruments marked with a scale, you get the last digit by estimating between the marks • if possible • Mentally divide the space into 10 equal spaces, then estimate how many spaces over the indicator is 1.2 grams Tro's Introductory Chemistry, Chapter 2

  22. Skillbuilder 2.3 – Reporting the Right Number of Digits • A thermometer used to measure the temperature of a backyard hot tub is shown to the right. What is the temperature reading to the correct number of digits? 103.4°F Tro's Introductory Chemistry, Chapter 2

  23. Significant Figures • Significant figures tell us the range of values to expect for repeated measurements • The more significant figures there are in a measurement, the smaller the range of values is; the more precise. 12.3 cm has 3 sig. figs. and its range is 12.2 to 12.4 cm 12.30 cm has 4 sig. figs. and its range is 12.29 to 12.31 cm Tro's Introductory Chemistry, Chapter 2

  24. Counting Significant Figures • All non-zero digits are significant • 1.5 has 2 sig. figs. • Interior zeros are significant • 1.05 has 3 sig. figs. • Trailing zeros after a decimal point are significant • 1.050 has 4 sig. figs. Tro's Introductory Chemistry, Chapter 2

  25. Counting Significant Figures • Leading zeros are NOT significant • 0.001050 has 4 sig. figs. • 1.050 x 10-3 • Zeros at the end of a number without a written decimal point are ambiguous and should be avoided by using scientific notation • if 150 has 2 sig. figs. then 1.5 x 102 • but if 150 has 3 sig. figs. then 1.50 x 102 Tro's Introductory Chemistry, Chapter 2

  26. diameter of a circle 2 radius of a circle = Significant Figures and Exact Numbers • Exact Numbers have an unlimited number of significant figures • A number whose value is known with complete certainty is exact • from counting individual objects • from definitions • 1 cm is exactly equal to 0.01 m • from integer values in equations • in the equation for the radius of a circle, the 2 is exact Tro's Introductory Chemistry, Chapter 2

  27. Example 2.4 – Determining the Number of Significant Figures in a Number • How many significant figures are in each of the following numbers? 0.0035 1.080 2371 2.97 × 105 1 dozen = 12 100,000 Tro's Introductory Chemistry, Chapter 2

  28. Example 2.4 – Determining the Number of Significant Figures in a Number • How many significant figures are in each of the following numbers? 0.0035 2 sig. figs. – leading zeros not sig. 1.080 4 sig. figs. – trailing & interior zeros sig. 2371 4 sig. figs. – all digits sig. 2.97 × 1053 sig. figs. – only decimal parts count sig. 1 dozen = 12 unlimited sig. figs. – definition 100,000 ambiguous Tro's Introductory Chemistry, Chapter 2

  29. Multiplication and Division with Significant Figures • When multiplying or dividing measurements with significant figures, the result has the same number of significant figures as the measurement with the fewest number of significant figures; round final answer: 5.02 × 89,665 × 0.10 = 45.0118 = 45 3 sig. figs. 5 sig. figs. 2 sig. figs. 2 sig. figs. 5.892 ÷ 6.10 = 0.96590 = 0.966 4 sig. figs. 3 sig. figs. 3 sig. figs. Tro's Introductory Chemistry, Chapter 2

  30. Rules for Rounding • When rounding to the correct number of significant figures, if the number after the place of the last significant figure is • 0 to 4, round down • drop all digits after the last sig. fig. and leave the last sig. fig. alone • add insignificant zeros to keep the value if necessary • 5 to 9, round up • drop all digits after the last sig. fig. and increase the last sig. fig. by one • add insignificant zeros to keep the value if necessary Tro's Introductory Chemistry, Chapter 2

  31. Rounding • Rounding to 2 significant figures • 2.34 rounds to 2.3 • because the 3 is where the last sig. fig. will be and the number after it is 4 or less • 2.37 rounds to 2.4 • because the 3 is where the last sig. fig. will be and the number after it is 5 or greater • 2.349865 rounds to 2.3 • because the 3 is where the last sig. fig. will be and the number after it is 4 or less Tro's Introductory Chemistry, Chapter 2

  32. Rounding & Writing in Scientific Notation • Rounding to 2 significant figures • 0.0234 rounds to 0.023 or 2.3 × 10-2 • because the 3 is where the last sig. fig. will be and the number after it is 4 or less • 0.0237 rounds to 0.024 or 2.4 × 10-2 • because the 3 is where the last sig. fig. will be and the number after it is 5 or greater • 0.02349865 rounds to 0.023 or 2.3 × 10-2 • because the 3 is where the last sig. fig. will be and the number after it is 4 or less Tro's Introductory Chemistry, Chapter 2

  33. Rounding • rounding to 2 significant figures • 234 rounds to 230 or 2.3 × 102 • because the 3 is where the last sig. fig. will be and the number after it is 4 or less • 237 rounds to 240 or 2.4 × 102 • because the 3 is where the last sig. fig. will be and the number after it is 5 or greater • 234.9865 rounds to 230 or 2.3 × 102 • because the 3 is where the last sig. fig. will be and the number after it is 4 or less Tro's Introductory Chemistry, Chapter 2

  34. Determine the Correct Number of Significant Figures for each Calculation and Round and Report the Result • 1.01 × 0.12 × 53.51 ÷ 96 = 0.067556 • 56.55 × 0.920 ÷ 34.2585 = 1.51863 Tro's Introductory Chemistry, Chapter 2

  35. Determine the Correct Number of Significant Figures for each Calculation and Round and Report the Result • 1.01 × 0.12 × 53.51 ÷ 96 = 0.067556 = 0.068 • 56.55 × 0.920 ÷ 34.2585 = 1.51863 = 1.52 result should have 2 sf 7 is in place of last sig. fig., number after is 5 or greater, so round up 3 sf 2 sf 4 sf 2 sf result should have 3 sf 4 sf 1 is in place of last sig. fig., number after is 5 or greater, so round up 3 sf 6 sf Tro's Introductory Chemistry, Chapter 2

  36. Addition and Subtraction with Significant Figures • When adding or subtracting measurements with significant figures, the result has the same number ofdecimal places as the measurement with the fewest number of decimal places 5.74 + 0.823 + 2.651 = 9.214= 9.21 2 dec. pl. 3 dec. pl. 3 dec. pl. 2 dec. pl. 4.8 - 3.965 = 0.835 = 0.8 1 dec. pl 3 dec. pl. 1 dec. pl. Tro's Introductory Chemistry, Chapter 2

  37. Determine the Correct Number of Significant Figures for each Calculation and Round and Report the Result • 0.987 + 125.1 – 1.22 = 124.867 • 0.764 – 3.449 – 5.98 = -8.664 Tro's Introductory Chemistry, Chapter 2

  38. Determine the Correct Number of Significant Figures for each Calculation and Round and Report the Result • 0.987 + 125.1 – 1.22 = 124.867 = 124.9 • 0.764 – 3.449 – 5.98 = -8.664 = -8.66 result should have 1 dp 8 is in place of last sig. fig., number after is 5 or greater, so round up 3 dp 1 dp 2 dp 6 is in place of last sig. fig., number after is 4 or less, so round down result should have 2 dp 3 dp 3 dp 2 dp Tro's Introductory Chemistry, Chapter 2

  39. Both Multiplication/Division and Addition/Subtraction with Significant Figures • When doing different kinds of operations with measurements with significant figures, do whatever is in parentheses first, find the number of significant figures in the intermediate answer, then do the remaining steps 3.489 × (5.67 – 2.3) = 2 dp 1 dp 3.489 × 3.4 = 12 4 sf 1 dp & 2 sf 2 sf Tro's Introductory Chemistry, Chapter 2

  40. Basic Units of Measure • The Standard Units: Scientists agreed on a set of international standard units called the SI units • Système International = International System Tro's Introductory Chemistry, Chapter 2

  41. Some Standard Units in the Metric System Tro's Introductory Chemistry, Chapter 2

  42. Length • Measure of a single linear dimension of an object, usually the longest dimension • SI unit = meter • About 3½ inches longer than a yard • Commonly use centimeters (cm) • 1 m = 100 cm • 1 cm = 0.01 m = 10 mm • 1 inch = 2.54 cm (exactly) Tro's Introductory Chemistry, Chapter 2

  43. Mass • Measure of the amount of matter present in an object • SI unit = kilogram (kg) • about 2 lbs. 3 oz. • Commonly measure mass in grams (g) or milligrams (mg) • 1 kg = 2.2046 pounds, 1 lb. = 453.59 g • 1 kg = 1000 g = 103 g, • 1 g = 1000 mg = 103 mg • 1 g = 0.001 kg = 10-3 kg, • 1 mg = 0.001 g = 10-3 g Tro's Introductory Chemistry, Chapter 2

  44. Related Units (Prefixes) in the SI System • All units in the SI system are related to the standard unit by a power of 10 • The power of 10 is indicated by a prefix • Prefixes are used for convenience in expressing very large or very small numbers • The prefixes are always the same, regardless of the standard unit Tro's Introductory Chemistry, Chapter 2

  45. Common Prefixes in the SI System Tro's Introductory Chemistry, Chapter 2

  46. Prefixes Used to Modify Standard Unit • kilo = 1000 times base unit = 103 • 1 kg = 1000 g = 103 g • deci = 0.1 times the base unit = 10-1 • 1 dL = 0.1 L = 10-1 L; 1 L = 10 dL • centi = 0.01 times the base unit = 10-2 • 1 cm = 0.01 m = 10-2 m; 1 m = 100 cm • milli = 0.001 times the base unit = 10-3 • 1 mg = 0.001 g = 10-3 g; 1 g = 1000 mg • micro = 10-6times the base unit • 1 m = 10-6 m; 106m = 1 m • nano = 10-9 times the base unit • 1 nL = 10-9L; 109 nL = 1 L Tro's Introductory Chemistry, Chapter 2

  47. Volume • Measure of the amount of three-dimensional space occupied • SI unit = cubic meter (m3) • a Derived Unit • Solid volume usually measured in cubic centimeters (cm3) • 1 m3 = 106 cm3 • 1 cm3 = 10-6 m3 = 0.000001 m3 • Liquid or gas volume, in milliliters (mL) • 1 L = 1 dL3 = 1000 mL = 103 mL • 1 mL = 0.001 L = 10-3 L • 1 mL = 1 cm3 Tro's Introductory Chemistry, Chapter 2

  48. Common Units and Their Equivalents Tro's Introductory Chemistry, Chapter 2

  49. Common Units and Their Equivalents Tro's Introductory Chemistry, Chapter 2

  50. Use Table of Equivalent Units to Determine Which is Larger • 1 yard or 1 meter? • 1 mile or 1 km? • 1 cm or 1 inch? • 1 kg or 1 lb? • 1 mg or 1 mg? • 1 qt or 1 L? • 1 L or 1 gal? • 1 gal or 1000 cm3? Tro's Introductory Chemistry, Chapter 2

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